Optical wavelength-division multiplexing transmission technique in optical fiber communication is of importance in order to economize a communication system and an optical multi/demultiplexer is an essential device for the optical wavelength-division multiplexing transmission.
Discrete type optical multi/demultiplexers such as an interference filter type and a grating type have been predominant in the past as the structure of the optical multi/demultiplexers. Studies have been made on a waveguide type structure, on the other hand, in order to accomplish a lower cost of production and one-chip monolithic type device. Mention can be made of an optical directional coupler type demultiplexer described in "1978 National Conference On EECE, Paper S6-2" by Sasaki and Ohguro, as an example of such prior art technique. According to this prior art reference, two optical waveguides a and b, each having a width W, are juxtaposed in a length L with a spacing S between them as shown in FIG. 7 of the accompanying drawings. Symbol n.sub.1 represents the refractive index of each optical waveguide and n.sub.2 represents the refractive index of the other portion. This prior art obtains the optical branching characteristics by tilizing the wavelength dependence of coupling of these two optical waveguides. According to this arrangement, two-wavelength multiple light Pi entering from a port 1 is isolated to light P.sub.1 having a wavelength .lambda..sub.1 and light P.sub.2 having a wavelength .lambda..sub.2 and they outgo from ports 2 and 3, respectively. FIG. 8 shows another structure which consists of a plurality of directional couplers 4-1 .about. 4-3 and a plurality of optical multiplexers 5-1 .about. 5-2 in order to improve the isolation characteristics.